The present invention relates to discharge lamps having a light emitting plasma contained within an arc tube. More particularly, the present invention relates to such discharge lamps having a coating applied to the inner wall of the arc tube to reduce or prevent chemical reaction between the discharge medium and the arc tube wall material or loss of fill material through the arc tube wall.
Discharge lamps such as mercury lamps, metal halide lamps, and high pressure sodium lamps include an arc tube forming a light emitting chamber containing the light emitting plasma. Typically the arc tubes are formed from a vitreous material such as fused silica and hard glass, or a ceramic material. The lamp fill material varies among the lamp types. For example, a metal halide lamp includes a fill of an inert gas, mercury, and one or more metal halides. During operation of the lamp, the fill material forms a light emitting plasma contained within the arc tube.
The useful life of such lamps is often reduced due to chemical reactions between elements in the discharge medium and the material forming the wall of the arc tube or loss of fill material through the arc tube wall. Such phenomena may cause electrode meltback, loss of fill material, and darkening of the arc tube wall which may result in a lumen loss over time, a shift in color, or a poor maintenance of light output.
For example, in a metal halide lamp having a sodium halide in the fill material, the useful life of the lamp can be limited by the loss of the metallic portion of the metal halide fill during lamp operation. This loss of metallic fill can be due to reaction of the metal halides with inner wall of the fused silica (SiO2) arc tube and/or sodium ion diffusion/migration through the wall of the arc tube. Both mechanisms result in an increase of free halogen in the arc tube. The term “free halogen” generally refers to volatile forms of halogens such as iodine or mercuric iodide.
The chemical reaction of the metal halides in the fill with the silica (SiO2) at the inner surface of the fused silica arc tube may produce metal silicate crystals and silicon tetraiodide. Silicon tetraiodide rapidly decomposes at the tungsten electrode tip which incorporates silicon in the tungsten electrode and releases free halogen. This results in a color shift, wall darkening, and lumen loss in the lamp.
Sodium ions of the fill material (e.g., sodium iodide (NaI)) are very mobile and may diffuse or migrate (under the influence of an external electric field) through the silica arc tube wall. The sodium ions are neutralized at the outer wall surface and may then condense on the outer lamp envelope. The halogen (e.g., iodine) component of the fill material does not diffuse through the arc tube wall and thus accumulates in the arc tube as free halogen. The lost sodium is thus unavailable to the discharge and can no longer contribute its characteristic emission. As a consequence of the loss of sodium and the build-up of free halogen, the light output gradually diminishes and the color shifts from white to blue. The arc becomes constricted and, in a horizontally-operating lamp particularly, may bow against the arc tube wall and soften it. Also, loss of sodium causes the operating voltage of the lamp to increase. The voltage increase brings about a rise in temperature to the point where the arc can no longer be sustained.
To counter these effects, conventional art suggests various methods. For example, U.S. Pat. No. 5,742,126 to Fuji et el. proposes coating the inner layer of the arc tube with one or more oxynitride layers of Al, Ta, Nb, V, Cr, Ti, Zr, Hf, Yb, Sc, Mg, Li and La. U.S. Pat. No. 5,668,440 to Inukai et al. discloses forming a barrier nitride layer by replacing the oxygen atoms of the quartz (SiO2) with nitrogen atoms to form silicon nitride (Si3N4). U.S. Pat. No. 5,394,057 to Russell et al. proposes applying a coating of metal silicate on the inside surfaces of the arc tube.
However, many of the conventional methods still fail to successfully prevent substantial lumen loss with lamp usage. Thus, there remains a need for chemically-stable barrier compositions that prevent, or at the very least inhibit, the extent of detrimental chemical reactions between fill constituents and the arc tube walls or the loss of fill material through the walls.
Therefore, it is an object of the present invention to provide a barrier coating and method for arc tubes that obviates the deficiencies of the prior art.
It is another object of the present invention to provide a barrier coating for arc tubes that is chemically stable and substantially inert when exposed to the discharge medium during operation of the lamp.
It is another object of the present invention to provide a novel method for deposition of a barrier coating on the inner wall of an arc tube.
It is still another object of the present invention to provide a barrier coating that inhibits the detrimental chemical reactions between fill constituents and the arc tube walls.
It is still another object of the present invention to reduce darkening of the arc tube walls over the life of a discharge lamp.
It is yet another object of the present invention to provide a discharge lamp with reduced lumen loss and color shift over the life of the lamp.
It will be noted that although the present invention is illustrated in view of these and other objectives, the principles of the invention are not limited thereto and will include all applications of the principles set forth herein.
These and other objects can be realized by simultaneous reference with the following non-exhaustive illustrative embodiments in which like segments are numbered similarly.